1. Field of the Invention
This invention relates to gas lift apparatus and more particularly to side pocket mandrels for use in wells produced by gas lift techniques and to methods of constructing side pocket mandrels.
2. Description of the Prior Art
Generally, side pocket mandrels have been constructed with elongate tubular bodies having a window in the side thereof and a side pocket receptacle body welded in the window so that the receptacle bore is positioned inside the mandrel for receiving flow control devices while a portion of the receptacle body is exposed through the window and is provided with lateral communication ports for conducting fluid flow between the receptacle bore and the exterior of the mandrel. Such construction requires welds in which at least a portion thereof extends longitudinally of the mandrel. Examples of such construction are illustrated in U.S. Pat. Nos. 2,824,525, 2,948,341, 3,412,806, 3,666,012, 2,942,671, 3,040,814, 3,606,393, 3,727,684, 3,741,299, 3,827,489, 3,994,339, 4,106,563, 3,802,503, 3,827,490, 4,030,543, 4,106,564, 3,807,498, Re. 29,870, 4,031,954, 4,135,576, 3,807,499, 3,874,445, 4,034,806, 4,146,091.
Some side pocket mandrels have utilized circumferential butt welds in their construction to join adjacent tubular members together. Examples of such welds are seen in U.S. Pat. Nos. 3,788,397, 3,827,490, 4,135,576, 4,146,091. But such circumferential welds are simply butt welds which join adjacent tubular members in end-to-end relationship.
Some side pocket mandrels are provided with deflector means extending above the receptacle bore for deflecting ordinary well tools moving through the mandrel back into the main flow passage to avoid possibility of lodging on or in the side pocket receptacle and yet permitting and even guiding a flow control device into the receptacle, thus facilitating its installation. Some of these deflectors are integral with the receptacle body, and some are welded to the upper end thereof. Most of the deflectors which are thus attached to the receptacle body tend to result in a longer window in which it is welded by a longer seam, this extra length being added to the longitudinal dimension of the seam.
Over the many years that such mandrels have been in use, many failures have occurred. Most of these failures occurred at longitudinal structural welds. Most of the weld failures occurred in the longitudinal portion of the seam. The term "structural weld," as used herein, denotes weld seams which form a structural part of the side pocket mandrels. These structural welds are subject to stresses created in the mandrels as a result of forces applied thereto as a result of tensile, columnal, or torsional loads, and are subject to pressures inside and outside the mandrels. Failure of such weld will at least cause a leak in the mandrel. Thus, the term "structural weld" as used herein also includes plug welds.
Failure in such welds is more readily understood in mandrels which are non-circular in cross section. Non-circular mandrels, which include the oval shaped and the flattened, appeared long ago, and for the last 10-15 years a very high percentage of the mandrels in use are non-circular. These are desired over the circular ones because two such mandrels will fit side by side or can pass each other in a smaller well casing, other things being equal. Or, alternatively, larger non-circular mandrels can be used in dual wells for some given casing sizes.
When side pocket mandrels, and especially non-circular ones, having longitudinal structural weld seams in the wall thereof, are subjected to high internal or external pressures, the lateral walls tend to expand or collapse, as the case may be. This expansion or collapsing movement of the mandrel wall results in very high stress concentrations at the longitudinal welds. It is extremely difficult, if not impossible, to obtain welds in such places that (1) do not have a notch at the inner side where the inner wall of the mandrel meets the outer wall of the receptacle body, and (2) do have full weld penetration.
Therefore, longitudinal structural welds provide weak places in such mandrels and are a source of early failure under severe or even ordinary conditions.
Also, many side pocket mandrels have been equipped with deflectors which have been joined to the mandrel through use of plug welds. In a similar manner, many mandrels have been equipped with orienting sleeves for orienting kickover tools, which sleeves have often been attached inside the mandrel by plug welds. Plug welds, while often used in the manufacture of side pocket mandrels, seldom appear in patents. What appear to be plug welds appear in U.S. Pat. No. 3,827,490.
Plug welds have also caused many failures. Since such welds are accomplished by welding a part such as a deflector or orienting sleeve to the inner wall through a hole in the mandrel wall, such hole resembles a window and at least a portion of the seam around the window will be longitudinal in direction relative to the mandrel. Thus, it is believed that failure occurs at a plug weld for the same reasons that failure occurs at a window. While the hole for the plug weld is smaller than the window in the window weld, the notch effect at the weld is somewhat similar.
U.S. Pat. No. 3,606,393 shows a receptacle body which has been made especially heavy in an effort to strengthen the window area and reduce weld failure, the increased mass of the material in the receptacle body being intended to prevent warping as a result of the welding, but the notch effect is still attendant, together with wall flexure under pressure in burst or collapse, and failures still occur.
Warpage of the mandrel as a result of welding has also been a problem in the manufacture of side pocket mandrels. This occurs, of course, because the hot weld metal shrinks upon cooling. Shrinkage is largely dependent upon the quantity of weld metal deposited. Thus, long axially extending seams warp more than short ones. When these seams are not opposite one another but are closer to one side of the mandrel, as they generally are in the case of a lateral window, especially a long narrow window, warpage can be considerable.
The stresses induced into the mandrel body by shrinking weld seams can literally destroy the mandrel, but these stresses can be relieved by hearing the entire mandrel in a process known as normalizing. This process does not, however, remove the warpage, and the mandrel must be straightened by bending in a direction opposite that of the warp. This straigtening process leaves the mandrel, then, with certain residual stresses therein, but, comparatively speaking, it is operable and is far more acceptable than before.
It should be understood that while a certain amount of stresses may be acceptable in a mandrel, it would be ideal to eliminate them entirely since they tend to facilitate certain corrosion processes.
U.S. Pat. No. 3,086,593 to Chitwood shows a check valve disposed in the flow passage extending through a gas lift valve which is installed in the offset receptacle of a side pocket mandrel to prevent back flow therethrough. Such check valves have been in common use. Similar check valves have been incorporated in such devices which are check valves per se and are not gas lift valves. These valves, when removed from the receptacle, leave the lateral ports thereof open for free movement of fluids therethrough in either direction.
Prior art drawing, Drawing No. 211--134 of Otis Engineering Corporation, P.O. Box 34380, Dallas, Texas 75234, shows a check valve disposed in a flow passage which is in direct communication with the extreme lower end of the offset receptacle of a side pocket mandrel. This check valve is not removable and, therefore, remains effective at all times, even when the receptacle bore is vacant, but this type of side pocket mandrel has very limited application because it is not compatible with conventional gas lift valves. In this type of mandrel, the lower end of the receptacle bore communicates with the exterior of the mandrel; in conventional mandrels, the lower end of the receptacle bore communicates with the main bore of the mandrel. In conventional gas lift valves for side pocket mandrels, lift gas enters the side of the valve and exits through its lower end into the lower portion of the receptacle bore. In the conventional side pocket mandrel, the receptacle bore is always drained and gas lift valves can be installed therein without difficulty. In mandrels where the lower end of the receptacle bore communicates with the exterior of the mandrel and a check valve is disposed in such communication passage, as in Otis Engineering Drawing No. 211--134 mentioned above, liquids can collect in the receptacle bore and the check valve will prevent their being drained, and installation of gas lift valves or other devices may be difficult or impossible because of such trapped liquids.
None of the prior art known to applicants shows a check valve in the lateral port of a side pocket mandrel, the lateral port being in direct communication with the receptacle bore at a location intermediate its ends, that is, at its mid section where devices can seal both above and below such lateral port.
The present invention overcomes the problems and shortcomings discussed above by providing side pocket mandrels and methods for their construction in which longitudinal structural weld seams, including window welds and plug welds, are eliminated entirely and warpage and straightening are minimized while providing a product which is constructed with ease and is stronger and less susceptible to failure. This invention also overcomes the check valve problem by providing check valve means in the lateral ports of the mandrels to control back flow through the lateral ports.